Field of the Invention
This invention relates to a method of making heteropoly acid compound catalysts containing oxides of molybdenum, vanadium, phosphorus, bismuth, copper, antimony, boron, cesium and, optionally, other metals. The catalyst may be used in a process for the oxidation of unsaturated aldehydes, such as methacrolein, to unsaturated carboxylic acids, such as methacrylic acid, in a vapor phase reaction.
Description of the Prior Art
Heteropoly acid compounds are metal oxide clusters forming heteropolyoxoanions in acid form and can include oxides of molybdenum, phosphorus, arsenic, cesium, rubidium, cobalt, nickel, iron, chromium, antimony, tellurium and silicon. Heteropoly acid compounds have a central metal atom surrounded by a framework of other metal atoms connected to each other and the central metal atom through oxygen atoms. The central metal atom is different (“hetero”) from the framework metal atoms. Heteropoly acid compounds are known as catalysts. The oxidation state of certain components has been shown to affect catalyst performance.
U.S. Pat. No. 6,914,029 discloses a selective hydrocarbon partial oxidation catalyst to partially oxidize C3 and C4 hydrocarbon selectively to acrylic acid and maleic acid. The catalyst is a partially reduced polyoxometallate, such as niobium polyoxomolybdate, prepared from a polyoxoanion which has been exchanged with a cation and activated by heating in the presence of a reducing agent, such as pyridinium. The cation should not be in its lowest oxidation state since it should be capable of partial reduction during activation. Changes in the oxidation state of the molybdenum were observed during activation and under reaction conditions.
U.S. Pat. No. 5,714,429 discloses a heteropolyacid catalyst supported on silica in the form of extrudates or pellets useful in a process for the hydration of olefins to corresponding alcohols. Polyvalent oxidation states of the heteropolyacids may change during the impregnation of the heteropolyacid onto the support and/or under process conditions.
Prior art discloses catalysts which contain molybdenum, vanadium, phosphorus, cesium, copper, bismuth, antimony, boron and other metals for the production of methacrolein. In the present invention, the oxidation state of certain components, specifically molybdenum and antimony, and the change of oxidation state of certain components during the process of making the catalyst are shown to affect the activity and stability of these catalysts for the oxidation of methacrolein to methacrylic acid.